The present invention relates to golf balls, more particularly to golf ball core compositions comprising blends of polybutadiene rubbers having improved properties and processability.
Conventional golf balls can be divided into two general classes: solid and wound. Solid golf balls include one-piece, two-piece (i.e., solid core and a cover), and multi-layer (i.e., solid core of one or more layers and/or a cover of one or more layers) golf balls. Wound golf balls typically include a solid, hollow, gas-filled, gel-filled or fluid-filled center, surrounded by a tensioned elastomeric material, and a cover. Solid balls have traditionally been considered more durable than wound balls, but many solid constructions lack the preferred xe2x80x9cfeelxe2x80x9d provided by the wound construction.
By altering ball construction and composition, manufacturers can vary a wide range of playing characteristics, such as compression, velocity, feel, and spin, each of which can be optimized for various playing abilities. In particular, a variety of core and cover layer(s) constructions, such as multi-layer balls having double cover layers and/or dual core layers, have been investigated. These golf ball layers are typically constructed with a number of polymeric compositions and blends, including, but not limited to, polybutadiene rubber, polyurethanes, polyamides, and ethylene-based ionomers.
The core of solid golf balls is the xe2x80x9cenginexe2x80x9d of the ball, providing the velocity required for good distance. Too hard a core, however, can result in a golf ball that provides poor feel. Manufacturers are constantly experimenting with various core compositions and constructions in an effort to optimize both feel and distance. Most conventional solid cores comprise polybutadiene rubber (xe2x80x9cBRxe2x80x9d) or some modified form thereof, which provides the primary source of resiliency for the golf ball.
BR core compositions still have room for improvement in resilience, which is determined by coefficient of restitution (xe2x80x9cCORxe2x80x9d). Familiar to those skilled in the golf ball art, the COR along with angle of trajectory (i.e., launch angle) and clubhead speed determine the distance a golf ball will travel when hit by a golf club. One way to measure the COR is to propel a ball at a given speed against a hard massive surface and measure its incoming and outgoing velocity. The COR is the ratio of the outgoing velocity to the incoming velocity and is expressed as a decimal between zero and one. There is no United States Golf Association limit on the COR of a golf ball, but the initial velocity of the golf ball cannot exceed 250xc2x15 feet/second. As a result, the industry goal for initial velocity is 255 feet/second, and the industry strives to maximize the COR without violating this limit.
In general, BR""s of high molecular weight (high Mooney viscosity) have better resilience than BR""s of low molecular weight (low Mooney viscosity). However, as the molecular weight increases, the milling and processing properties of the BR deteriorate. BR catalyzed with lanthanide series elements such as neodymium tends to be linear and narrow in polydispersity (close to 1.0). The narrow polydispersity allows high-molecular weight Ndxe2x80x94BR of to process readily, but the linearity causes problems in extrusion processes such as die swell and cold flow. BR catalyzed with cobalt and/or nickel, in comparison to Ndxe2x80x94BR, tends to be more branched and have wider polydispersity (distant from 1.0). While the branching characteristic facilitates processing, the wide polydispersity generally gives low resilience. Advantageously, blends of Co/Nixe2x80x94BR and Ndxe2x80x94BR in core compositions enhance resilience in the resulting golf balls.
Attempts to improve golf ball COR by using various blends of BR in core compositions include, among others, U.S. Pat. Nos. 4,683,257; 4,931,376; 4,955,613; 4,984,803; 5,082,285; 6,139,447; 6,277,920; and 6,315,684. Although some of the core compositions described in these disclosures are satisfactory, a need remains for compositions with improved properties and processabilility to form golf balls.
The present invention is directed to a golf ball having a solid core and a cover. The solid core is formed from a blend of two polybutadiene rubbers, one made with a cobalt or nickel catalyst and having a higher Mooney viscosity between about 50 and about 150, another made with a lanthanide series catalyst and having a lower Mooney viscosity between about 30 and about 100. A neodymium catalyst is a preferred lanthanide series catalyst. The blend has more of the Co/Nixe2x80x94BR and less of the Ndxe2x80x94BR, with a ratio of weight percentage between the two being preferably at least about 51:49, more preferably at least about 60:40, and most preferably at least about 75:25.
Preferably, the Co/Nixe2x80x94BR has a Mooney viscosity between about 60 and about 150, a number average molecular weight between about 150,000 and about 250,000, and a polydispersity between about 1.50 and about 3.50, while the Ndxe2x80x94BR has a Mooney viscosity between about 35 and about 90, a number average molecular weight between about 150,000 and about 275,000, and a polydispersity between about 1.25 and about 2.75. More preferably, the Mooney viscosity of the Co/Nixe2x80x94BR is between about 70 and about 130, and the Mooney viscosity of the Ndxe2x80x94BR is between about 45 and about 80. The polybutadiene blend also has a cis-1,4 bond content of at least about 80% in the polymer chains, and it comprises preferably at least about 65% by weight of the golf ball core, more preferably between about 70% and about 85%.
One conventional adduct to the BR blend is a reactive co-agent that crosslinks (i.e., vulcanizes) the BR. To prevent water absorption to the core and enhance the durability of the golf ball, the amount of co-agent is minimized to preferably less than about 10 parts per hundred (xe2x80x9cphrxe2x80x9d) by weight of the BR blend, more preferably less than about 5 phr, and most preferably about 0 phr (no co-agent). Alternatively, a moderately hard core may be achieved using the co-agent in an amount between about 10 phr and about 50 phr by weight of the BR blend. Furthermore, the core may be very hard and dense when at least about 50 phr of the co-agent is incorporated into the core. The co-agent is preferably a salt of an unsaturated carboxylic acid having about 3 to 8 carbon atoms; an unsaturated vinyl compound; a polyfunctional monomer; phenylene bismaleimide; or a mixture thereof.
Besides the Co/Nixe2x80x94BR and Ndxe2x80x94BR blend, the solid golf ball core may further comprise other species of BR, as well as natural rubber; balata; gutta-percha; synthetic polyisoprene; styrene-butadiene rubber; styrene-propylene-diene rubber; chloroprene rubber; acrylonitrile rubber; acrylonitrile-butadiene rubber; ethylene-propylene rubber; ethylene-propylene-diene terpolymer; and mixtures thereof. Preferably, the core has an outer diameter of about 1.40 inches to about 1.65 inches, and the cover has a thickness of about 0.01 inches to about 0.20 inches. The core may comprise a center and at least one outer core layer, at least one of which comprises the Coxe2x80x94BR and Ndxe2x80x94BR blend. The cover may comprise one or more layers including inner cover layer and outer cover layer.
The present invention is also directed to a multi-layer golf ball comprising a core, a cover, and at least one intermediate layer disposed between the core and the cover. The intermediate layer preferably comprises an elastomeric composition having a reactive co-agent in an amount less than about 10 phr by weight of the elastomer, so that the intermediate layer has moisture vapor barrier properties. Specifically, the intermediate layer has a water vapor transmission rate of less than about 250 gxc2x7mil/100 in2xc2x724 h. The co-agent may be a salt of an unsaturated carboxylic acid having about 3 to 8 carbon atoms; an unsaturated vinyl compound; polyfunctional monomer; phenylene bismaleimide; or a mixture thereof. The core may be solid, hollow, gas-filled, gel-filled, or fluid-filled. The core may also comprise a center and at least one outer core layer.
The elastomeric composition of the intermediate layer comprises a BR blend of a Co/Nixe2x80x94BR with a higher Mooney viscosity of from about 50 to about 150, and a Ndxe2x80x94BR with a lower Mooney viscosity of from about 30 to about 100. The ratio of weight percentage between the two BR""s ranges from about 5:95 to about 95:5. Content of the co-agent may be between about 10 phr and about 50 phr by weight of the elastomer for moderate hardness, or at least about 50 phr for a dense intermediate layer.
For such a multi-layer golf ball, the core may have an outer diameter of about 0.25 inches to about 1.60 inches. The intermediate layer comprises one or more layers, preferably having an overall thickness between about 0.01 inches and about 0.60 inches, more preferably between about 0.02 inches and about 0.10 inches. The cover of the multi-layer golf ball may have a single layer, or multiple layers including at least one inner cover layer and an outer cover layer. The core may have a center and at least one outer core layer. Other suitable adducts for the elastomeric composition of the intermediate layer include natural rubber; balata; gutta-percha; cis-polybutadiene; trans-polybutadiene; synthetic polyisoprene; polyoctenamer; styrene-propylene-diene rubber; metallocene rubber; styrene-butadiene rubber; ethylene-propylene rubber; chloroprene rubber; acrylonitrile rubber; acrylonitrile-butadiene rubber; styrene-ethylene block copolymer; ethylene-propylene-diene terpolymer; maleic anhydride or succinate modified metallocene catalyzed ethylene copolymer; polypropylene resin; ionomer resin; polyamide; polyester; urethane; polyurea; chlorinated polyethylene; polysulfide rubber; flurocarbon; or a mixture thereof.
The present invention is further directed to a golf ball comprising a core, a cover, and at least one thin dense intermediate layer disposed between the core and the cover. This thin dense intermediate layer comprises an elastomeric composition having at least about 15 phr of a reactive co-agent, preferably at least about 50 phr, and at least one density-modifying filler to achieve a specific gravity of at least about 1.2. The thin dense intermediate layer has a thickness between about 0.01 inches and about 0.10 inches.